Machine translation (MT) concerns the automatic translation of natural language sentences from a first language (e.g., French) into another language (e.g., English). Systems that perform MT techniques are said to “decode” the source language into the target language.
One type of MT decoder is the statistical MT decoder. A statistical MT decoder that translates French sentences into English may include a language model (LM) that assigns a probability P(e) to any English string, a translation model (TM) that assigns a probability P(f|e) to any pair of English and French strings, and a decoder. The decoder may take a previously unseen sentence f and try to find the e that maximizes P(e|f), or equivalently maximizes P(e)·P(f|e).
A TM may not model structural or syntactic aspects of a language. Such a TM may perform adequately for a structurally similar language pair (e.g., English and French), but may not adequately model a language pair with very different word order conventions (e.g., English and Japanese).